I'm gonna stick with the "fourth race that the Vasari are running from" theory.
unlikely, its too cheezy/overdone.
I am wondering if there is a 4th tech tree of stolen tech that is modified for each race... Possibly only obtained via espionage?
back off, you know I came up with the reverse engineering idea first.
Maybe some kind of creation the made to keep track of all the races they subjugated.
I think they're just a bit more creative than that... but maybe not.
and besides, if it was just meant explicitely to fight that one thing, why would they bother mentioning it as "another tech tree" because it really isnt, its just a pathetic "research this to live!!!" tak on. I'm sticking with the reverse engineering.